Realization of a second-order node by means of two add units and two 1:2 splitter units as drop units is known. Corresponding structures are shown in FIGS. 1b and 1a, respectively. A drop unit 1 according to FIG. 1a can be realized by means of a 1:2 splitter unit 3 and a demultiplexing unit 5. The receive WDM signal (R-WDM) received by a remote port is fed to this drop unit. Because the splitter unit 3 generates WDM signals, which correspond to the R-WDM signal and have only a small optical power, at the output port, it is possible to realize a broadcast function, i.e., a channel CH1 to CHn can be dropped in the node and also transmitted simultaneously at another port. The signals of the individual channels are applied to the channel output ports of the demultiplexing unit 5 and can be processed further. The channel output ports here together form the output part LPout of the bidirectional local port LP and guide the (local-transmit) signals supplied from the local-port LP.
The add unit 7 shown in FIG. 1b is composed of a demultiplexing unit 9, a number of optical 2×1 switches 11 and variable optical attenuators 13 corresponding to the number of channels of the relevant WDM signal, as well as an optical multiplexing unit 15.
A cross-connect WDM signal (CC-WDM) fed to the add unit 7 at the input port 7a is fed to the input port 9a of the demultiplexing unit 9, which generates the signals of the individual channels at the demultiplexer channel output port. The demultiplexer channel output ports are each connected to one input port of a 2×1 switch 11. Each of the other input ports of the 2×1 switches 11 is connected to one local port or to a number of ports corresponding to the number of channels, to each of which the signal of an individual channel can be fed directly and which together form the input part LPin of the bidirectional local port LP (local receive, i.e., the signals fed to the local port).
Using the (preferably controllable) 2×1 switch, the circuit can select whether a certain channel of the CC-WDM signal or a relevant channel of the local port is output via the output port of the switch 11 to the variable attenuators 13 and fed via these elements to the channel input port of the multiplexing unit 15. The optical power carried in the respective individual channel signals can be held within a given range by means of the attenuators 13. For this purpose, the optical power carried in the channels is detected by means of detectors (not shown) and the attenuators are each controlled so that the channel signal power at each output of the attenuators lies in a given range. A transmit-WDM signal (or a sub-transmit-WDM signal), which for each channel CH1 to CHn can contain selectively either the channel signal of the CC-WDM signal or the relevant channel signal fed to the local port LP, can be supplied to the output port 15a of the multiplexing unit 15, which is connected to the output port 7b of the add unit.
Obviously, the demultiplexing unit 5 can be left out or contained in a downstream unit, if the split R-WDM signal can be fed directly to this unit. The local port LPout then guides a WDM signal. Analogous to this situation, a demultiplexing unit (not shown), which demultiplexes a local-receive WDM signal fed to the input port of the multiplexing unit into the individual channels, is provided at the local port LP of the add unit 7. Such a local-receive WDM signal can be delivered, for example, from a channel card that carries a number of tunable transmit elements whose output signals are combined to form a WDM signal.
FIG. 2 shows a schematic of an optical circuit structure for a second-order node that has two remote ports RP1 and RP2 which are composed of the logical sub-ports “RP1in” and “RP1out” and “RP2in” and “RP2out”, respectively. In practical optical networks, either a separate optical waveguide can be used for each transmission direction or only one optical waveguide is used, which is then bidirectional.
The circuit structure shown in FIG. 2 for each remote port RP1 and RP2 is composed of a 1:2 splitter unit 3 and an add unit 7, as well as a unit 17 for demultiplexing each split receive WDM signal R-WDM1 and R-WDM2, respectively, into the individual channels CH1 to CHn and for the further processing of the actual channels to be dropped. The demultiplexing unit 5 integrated in FIG. 1 into the drop unit 1 is contained in the unit 17 in the embodiment according to FIG. 2.
In addition, the circuit structure according to FIG. 2 includes for each remote port RP1, RP2 a transmit unit 19, which generates the signals of each channel CH1 to CHn to be integrated into the transmit WDM signal T-WDM1 or T-WDM2 to be supplied from the relevant remote port RP1, RP2. An optical amplifier 21 is also provided on the input and output of each sub-remote port RP1in, RP1out and RP2 in, RP2out, in order to first amplify the incoming signals and to compensate for the losses for the transmitted signals in the circuit structure of the node.
Sub-structures for realizing higher-order nodes for complex optical networks, so-called wavelength selective switches (WSS), have been developed in recent years. A WSS here is composed of several equal-access input ports, to each of which a demultiplexing unit is connected downstream. The demultiplexing outputs for the individual channels (to each of which is allocated a given center wavelength and a certain bandwidth) are each fed to one input port of an optical N×1 switch, with the number of switches corresponding to the number of channels of the WDM system to be realized. The switches each switch a channel of a certain input port of the WSS through to an output port of each switch. The output ports of the switches are connected to the input ports of a multiplexing unit, so that a WDM signal which contains channels of a certain input port in a given way is output at the output port of the multiplexing unit. A node of order N can thus be realized with N WSS, which are connected in a certain way to N optical 1:N splitter units.
An N-order node here has N remote ports, to each of which is fed a receive wavelength division multiplexed signal R-WDM and from each of which can be output a transmit wavelength division multiplexed signal T-WDM. A node with full cross-connect capability allows the extraction of an arbitrary channel of a receive WDM signal fed to an arbitrary remote port and the integration of this channel into the transmit WDM signal of another arbitrary remote port.
In addition, an N-order node typically has N local ports, with each local port being associated with a certain remote port. In this case, it is possible to supply each desired channel of the receive WDM signal of the associated remote port to the local port and to terminate the traffic relation realized by means of this channel in the node. In the same way, an arbitrary channel can also be fed to each local port, wherein this channel is then integrated into the transmit WDM signal of the associated remote port.
The WSS described briefly above, however, involves a relatively new technique, which has not yet been accepted without hesitation by all users, especially operators of large networks.